Geochemical prospecting



R. T. SANDERSON GEOCHEMICAL PROSPECIING May 15, 1945.

2 Sheets-Sheet 1 Filed Feb. 15, 1941 May 15, W45. R. T. SANDERSON2,375,949

GEOCHEMICAL PROSPECTING Filed Feb. 15, 1941 2 Sheets-Sheet 2 PatentedMay 15, 1945 GEOCHEMICAL PROSPECTING Robert Thomas Sanderson, Fishkill,N. Y., assignor to Stanolind Oil and Gas Company, Tulsa. Okla, acorporation of Delaware Application February 15, 1941, Serial No.379,109

12 Claims.

This invention relates to geochemical prospecting and more particularlyto a method and apparatus for geochemical prospecting which isparticularly adapted to use in the field.

A set of procedures known generally as geochemical prospecting has beendeveloped in recent years. In the main this prospecting technique has todo with the analysis of samples of soil, soil gas, or other geologicalcomponents for minute traces of substances indicative of underlyingmineral deposits. The samples are usually analyzed for minute traces oflight hydrocarbons, principally the gaseous paraflin hydrocarbonsheavier than methane, and in some instances the lighter normally liquidhydrocarbons, which are believed to seep upwards in gaseous or vaporform from underlying oil and gas deposits, and the presence of which inabnormally high quantities is indicative of the presence at greaterdepths of such oil and gas deposits. Other techniques using differentindicator substances are known but are generally conceded to be inferiorto those utilizing the lower paraflin hydrocarbons heavier than methane.

Geochemical prospecting has been conducted in the past by samplingeither the soil or the gases present in the soil and taking thesesamples to the laboratory where they are analyzed for indicatorsubstances by elaborate techniques unsuited to field use. The necessityfor taking samples to the laboratory is particularly burdensome whensoil gas or soil air samples, rather than samples of the soil itself,are used since the sample containers must necessarily be bulky andrelatively expensive, particularly in view of the fact that anygeochemical survey necessarily involves a large number of samples takenat spaced survey points. Largely for this reason many operators havegone to the use of soil samples rather than soil gas samples but thisprocedure is open to very serious question since the amount ofhydrocarbons sorbed on the particles of soil is aifected by many factorsand is not necessarily a true indication of the amount of hydrocarbonsmigrating upward through th soil from the underlying oil and gasdeposit. To be sure the results obtained by the use of soil samples canbe corrected to a considerable extent in various ways but thecorrections add to the cost and it is never completely certain thatentirely adequate corrections have been made.

An object of the present invention is to provide a method and apparatusfor geochemical v prospecting utilizing soil gas samples butavoidchemical prospecting which is simple and rapid.

Other and more detailed objects, advantages and uses of my inventionwill become apparent as the description thereof proceeds.

In short I construct at each sampling station an enclosed hole in thesoil and a suflicient amount of time is allowed to elapse to permit theair in this hole to come to substantial equilibrium with the soil gas.Apparatus of a type which will later be described is then put incommunication with the hole, preferably with a purification trainintervening, the apparatus is flushed out with gas withdrawn from thehole, and further gas is withdrawn from th hole and passed through coldtraps which serve to condense significant hydrocarbons. Thesesignificant hydrocarbons are then evaporated by warming them andflushing them with further gas, preferably withdrawn from the hole, andthey are then passed to a measuring device.

My method and apparatus will be more fully described with reference tothe accompanying drawings which form a part of this specification andare to be read in conjunction therewith. In the drawings:

Figure 1 is a diagrammatic elevation of one form of apparatus inaccordance with my invention; and

Figure 2 is a diagrammatic elevation of an alternative type of apparatusin accordance with my invention.

Turning now to Figure 1 in more detail, a pit or enlarged hole ill isvfirst formed in the soil and from the bottom of this pit a hole i l isdrilled in the soil. Pit Hi can, for instance, be two or three feet longand at least a foot or two deep, while hole H can be from two to sixinches in diameter, preferably about 4 inches in diameter, and from 5 to50 feet deep; for instance 10 or 15 feet deep. Pits I U and holes ii aredug in the desired locations, for instance at sampling stations locatedone-tenth mile apart along a survey line or throughout a survey area,several days prior to sampling and are sealed at the top by insertingplug I2 carrying sampling tube l3 and packing pit it above plug i2 withmoist earth, clay, or other sealing material It. The top of tube l3 canat the time the hole is originally drilled be sealed with a rubber tubehaving the upper end closed or by other means. Sampling tube l3protrudes slightly above the surface of the ground and can be marked sothat it can later be located by the analysis crew. This sampling tubepreferably extends well downin hole so as to minimize further anypossible contamination of the sample withdrawn therethrough withatmospheric air.

After a sufficient time has elapsed to accomplish at least approximateequilibrium between the air in the hole and the soil gas, tube I3 isuncovered and the analytical apparatus is at tached by means ofconnector l5 as shown in Figure 1 or otherwise. The time required toaccomplish equilibrium is usually at least five days and longer periodsare preferable.

The apparatus shown in Figure 1 is particularly adapted to betransported by hand if necessary and can conveniently be mounted on oneor more panels to facilitate this. Thus, geochemical propecting can becarried out in relatively inaccessible locations without the necessityfor transporting bulky apparatus or bulky gas samples.

My apparatus, as shown in Figure 1, preferably includes a purificationsorption train made up of bulbs I611, I61) and |6c interconnected byconnectors Ila and I'll). This sorption train can suitably containmaterials for the removal of basic substances, acidic substances and thebulk of the water vapor. Thus bulb |6a can contain sodium bisulfate orother acidic material adapted to remove ammonia and other basicsubstances, bulb |6b can contain Ascarite (potassium hydroxide supportedon asbestos) or other alkaline material for removing acidic substancessuch as CO2, etc., and bulb |6c can contain a drying agent, for instancecalcium chloride. Other types of purification trains can, of course, beused and the purification train can, moreover, be omitted in someinstances.

Conduit H3 is attached to the purification train by connector I9 andleads to three-way stopcock which is adapted to connect conduit |8 withconduit 2| leading to the first trap 22a. of a trap system andalternatively to connect conduit 2| with conduit 23 leading to a gasdetection and measurement apparatus to be described later.

Trap 22a, and also trap 22b, is preferably of the coil type and cansuitably be Packed with glass wool or the like to make the trappingoperation as complete as possible. The two traps are connected byconduit 24 and from this or some other convenient part of the system,conduit 25 leads to manometer 26. From trap 22b conduit 21 leads throughthree-way stopcock 28, conduit 29 and connector 30 to bulb 3|, thepurpose of which will later be described. Conduit 32 containingthree-way stopcock 33 connects conduits 8 and 21 as shown.

Bulb 3| contains activated charcoal or other highly efiicient sorbentwhich is adapted to sorb much greater quantities of gas at a lowtemperature, such as liquid air or liquid nitrogen temperature, than athigher temperatures such as atmospheric temperature.

When the apparatus has been assembled, as shown in Figure 1, three-waystopcock 20 is positioned to connect conduit 2| and conduit 23 leavingthe former unconnected with conduit l8. Three-way stopcock 34 ispositioned to connect one of bulbs 35a and 35b with conduit 23.Stopcocks 36a and 361) are opened, connecting gas analysis chambers 31aand 31b with bulbs 35a and 351), respectively. Stopcock 33 is closed andstopcock 28 is positioned to connect bulb 3| with conduit 21 and toclose ofi vent conduit 38. with the apparatus in this condition a bath39d of liquid air or, better, liquid nitrogen, the latter being atapproximately 196 C., is placed around bulb 3|. The result is that theair present in the apparatus connected with bulb 3| is rapidly sorbedand the pressure quickly falls. If a good grade of activated charcoal isused in a quantity of about 50 to 100 grams it will sorb at 196 C. from10 to 25 liters of air or soil air in excess of the amount sorbed by itat atmospheric temperature. In other words, bulb 3| acts as a pump. Whenthe pressure has dropped to a low value, for instance less than 5 mm. ofmercury, stopcock 34 is turned to connect the unevacuated bulb 35a or3512 with conduit 23. When the pressure has again dropped to a low valuethe apparatus is evacuated and ready for use.

Stopcocks 28, 36a and 36b are now closed and stopcock 33 is turned toconnect conduit l8 with conduit 21 through conduit 32. Soil gas nowfiows through the purification train, conduit 32, the traps (atatmospheric temperature) and stopcocks 20 and 34 into one of bulbs 35aand 35b, say bulb 35a, which is thus filled with a sample of purifiedsoil gas. Stopcocks 20 and 33, and preferably also stopcock 34, are nowturned to the completely closed position.

Bath 39a, which can suitably be at a temperature in the neighborhood ofC., is now placed around trap 22a. This bath can, for instance, be anether-carbon dioxide bath or an acetone-carbon dioxide bath. Bath 39bcontaining liquid air or preferably liquid nitrogen, the latter being atapproximately -196 C., is placed around trap 221). At this pointthree-way stopcock 20 is turned to connect the traps with thepurification train or, in other words, to connect conduit l8 withconduit 2| and stopcock 28 is turned to connect pump 3| with conduit 21,

Soil gas from bore hole now enters through the sorption or purificationtrain, being freed of basic substances, acidic substances and most ofits water in the course of its progress through this train. The purifiedgases pass through trap 22a. at approximately -80 C; and trap 22b atapproximately -196 C. and on into the sorption bulb or pump 3|. Otherbath temperatures can be used.

As previously indicated, a very large volume of soil gas can be passedthrough the apparatus by means of sorption bulb 3 However, the totalvolume of gas so withdrawn from bore hole II should be considerably lessthan the total volume of that bore hole, for instance from 5% to 75% ofits volume and preferably from 15% to 40% of its volume.

It is also important that the conditions be maintained entirely constantat the various survey stations so that comparable results can beobtained in all instances. Thus the amount of material passed throughthe traps should be standardized for all survey stations. This resultcan be secured at least approximately by using a standardized quantityof a standardized activated charcoal and standardizing the pressures ineach step in the process as well as standardizing the dimensions of thehole from which the soil gas sample is withdrawn.

As the soil gases pass through trap 22a practically all substancesnon-volatile at -80 C. are condensed therein and, similarly, substancesvolatile at 80 C. and condensable at --l96 C. are removed by trap 22b.After five to ten minutes the sorption by the charcoal in pump 3| ispractically complete. Three-way stopcock 20 is then moved to completelyclosed position and threeway stopcock 28 is positioned to connectcharcoal bulb 38' with vent line 38. Bath 39d is removed from thecharcoal bulb permitting its temperature to rise to atmospheric withresultant desorption which puts the charcoal in condition for re-use.

At this point bath 39b is removed and trap 22b is thereby warmed.Three-way stopcock 20 is positioned to interconnect bulb 35b with trap22a and stopcock 33 is positioned to interconnect tubes l8 and El. Thisassumes that bulb35b is the evacuated bulb and that bulb 35:: contains asoil gas sample. Since bulb 35b is rather highly evacuated and theapparatus between three-way stopcocks 20 and 28 as well as thepurification train and the bore hole H are at a pressure appreachingatmospheric, further soil gas is drawn in through conduit l3, thepurification train and conduit 32, and passes through traps 22b and 22ain sequence. This soil gas or soil air" flushes hydrocarbons,principally an ethane-pentane fraction, originally condensed on theinner walls of trap 22b out therefrom, through trap 22a and into bulb35b. Stopcock 3A is then closed.

The sample is retained in bulb 35b long enough to warm to approximatelythe same temperature as that of the sample in bulb 35a.

The use of at least two traps is definitely advantageous since trap 22aserves to remove residual water and other materials which tend tointerfere with the determination of hydrocarbons. However, a single trap221) operating at a very low temperature such as the boiling point ofliquid air or, better, liquid nitrogen, can be used omitting trap 22a.When utilizing two traps any traces of materials non-volatile at --80 C.which may have been blown in mist or frozen spray form into trap 221; bythe soil gas current original-1y passed in succession through traps 22aand 22b is recondensed in trap 22b. Then when soil gas from conduit 32is passed in sequence through traps 22b and 22a. this materialnon-volatile at ,80 C. orwhatever the temperature of trap 2211. may be,does not reach bulb 35b and is not measured. Thus a determination ismade of an ethane-pentane hydrocarbon fraction (or other desiredhydrocarbon fraction, depending on the trap temperature chosen)uncontaminated with substances non-volatile at the temperature of trap22a.

The gas detecting part of the apparatus includes two platinum coils 40aand dob, or coils having a surface containing at least one platinumgroup metal, adapted to promote surface combustion. These two coils aredisposed in chambers 31a and 31b respectively. Along with resistancesdla and Mb and associated electrical connections, they form a Wheatstonebridge which can be balanced by adjustment of the relative value ofthese resistances. A potential, for instance that supplied by battery42, is applied to one diagonal of the bridge when switch 63 is closed.The other diagonal is connected through a galvanometer or milliammeterit. Battery d2 serves to heat coils Ma and tub.

Stopcocks 36a and 38b are now opened and gases from bulbs 35a and 35benter chambers 31a and 31b and impinge on heated coils 40a and 4102).The surface oxidation of the hydrocarbons contained in the gasesentering chambers 31a and 3'lb causes the temperatures of coils 40a and40b to increase, with resultant increase in their electricalresistances. Indicating device M gives a reading which is a measure ofthe relative temperature increases of coils 40a and 40b and thus ameasure of the relative amounts of soil hydrocarbons contained in thesamples of soil gas stored in bulbs 35:: and 3512. Bulb 35a contains theoriginal soil gas while that contained in bulb 35b contains a similarsample enriched by the heavier soil hydrocarbons trapped from a muchlarger volume of soil gas. Thus coil 40a measures a blank and balancethis out against a sample of the soil gas containing the trapped heavierhydrocarbons. Methane and other low boiling combustibles contained inthe soil gasare not measured by instrument 46 which only measures thesignificant volatile hydrocarbons heavier than methane.

This measure can by suitable experiments with known samples be convertedto give approximate values of hydrocarbon content in terms of parts permillion or parts per billion or in such other terms as may be desiredbut this is not essential since in geochemical prospecting all that isneeded is relative values for the various survey stations and thereadings of indicating instrument 44 at various survey stations can thusbe directly compared without any compelling need for converting theminto hydrocarbon concentration figures.

Instead of using a sample of soil gas in bulb 35a and using soil gas toflush the trapped hydrocarbons into bulb 35b, atmospheric air can beused for these purposes. Thus instead of filling bulb 3511 with soil gasstopcock 33 can be turned to connect this bulb with the atmosphere viaconduit 45 and air from this same conduit can be used to flush thetrapped hydrocarbons into bulb 35b.

The volume of bulb 35b (the same as that of bulb 35a) should preferablybe in excess of the volume of traps 22a and 22b and the remainder of theapparatus disposed between three-way stopcocks 20 and 28 so that aconsiderable volume of gas is drawn through the two traps, forcing atleast the bulk of the hydrocarbons initially trapped in trap 221) intobulb 35b and ultimately into contact with coil 40b.

It will be understood, of course, that the methods and apparatusdescribed are subject to considerable variation in detail and I haveillustrated certain modifications which are suitable in many instancesin Figure 2 in which parts corresponding to Figure l carryingcorresponding reference numbers. Various features of Figure 2 can beused in conjunction with the apparatus of Figure 1 and vice versa aswill be apparent to those skilled in the art.

In Figure 2 a board lZa and screen l2b are used to replace plug l2 ofFigure 1, and serve the purpose of keeping soil It or other sealingmaterial out of bore hole ll.

Figure 2 also illustrates a purification train using liquid purificationagents instead 'of solid purification agents as in Figure 1. Thusconduit 46 leads from connnector l5 to tightly stoppered bottles Ito andlfib, which can suitably contain concentrated sodium hydroxide solutionand concentrated sulfuric acid respectively and serve to replacepurification bulbs "5a, 661) and 460 of .Figure 1. v.

The trap system of Figure 2 is similar to that of Figure 1 except that athird trap 22c and bath 390 are added for purposes which willhereinafter be described.

In Figure 2 a different means for evacuating the system is provided andthis means is particularly suitable where the survey stations areaccessible to a small truck while the set-up of Figure 1 is particularlysuitable where hand carrying is necessary. Three-way stopcock 28 ofFigure 2 is connected to a vacuum chamber 41 to which manometer 48 isconnected. This vacuum chamber is connected through stopcock 49 with amotor driven vacuum pump 3|, dia grammatically indicated.

Vacuum chamber 41 is used to standardize the volume of soil gaswithdrawn from bore hole H. When the system including vacuum chamber 41,traps 22a, 22b and 220 and the gas detection device has been evacuatedby means of motor driven vacuum pump 3| to a predetermined low pressurelevel as indicated by manometer 48, stopcock 46 isclosed and the pump isturned off. From this point on the analysis is similar to that using theapparatus of Figure 1 and consists in connecting the trap system tovacuum chamber 47 by means of three-way stopcock 28 and to thepurification system by means of three-way stopcock 20. Thus soil gas isdrawn through the purification system and the trap system into thevacuum chamber and the amount of this gas thus drawn through the systemcan be standardized for each survey station by simply evacuating thesystem in the first instance to the same pressure so that with aconstant volume vacuum chamber 41 and standardized initial vacuum inthis chamber the volume of gas withdrawn before the pressure increasesto atmospheric or some other predetermined level (as indicted bymanometer 26 or 48) is necessarily a constant.

The three traps of Figure 2 permit a more eificient fractionation of thehydrocarbons than do the two traps of Figure l. The procedure with thethree trap system is the same as with the two trap systems untilstopcock 33 has :been positioned to connect conduit 27 with conduit 18and the pressure in the trap system has increased to about /2 atmosphereas shown by manometer 26. Stopcock 33 is then closed and bath 39a at -80C., or other temperature of that order, is removed from about trap 22aand bath 390, which can be at this same temperature, is placed abouttrap 22c. As a matter of fact,'baths 39a and 390 can be the same sincethey are not used simultaneously in the method under description. Whentrap 22a has warmed to approximately atmospheric temperature, stopcock33 is again positioned to connect conduits l8 and 2'! until the systemcomes to atmospheric pressure or some predetermined pressure levelapproaching atmospheric. The purpose of this redistillation procedure isto free any traces of ethane-pentane hydrocarbons which may have beentrapped in trap 22a.

The gas detection or measurement apparatus of Figure 2 difiers somewhatfrom that of Figure 1. While that of Figure l is preferable so far asaccuracy is concerned, that of Figure 2 simplifies the proceduresomewhat. In the first place, an enlarged bulb 50 connected with the gasdetection apparatus 'by conduit can be evacuated initially along withthe rest of the system and when gas is admitted to the gas measurementapparatus the bulb increases the volume of gas drawn through the systemand makes certain that the hydrocarbons reach the combustion coils. Thevolume of the evacuated gas measurement apparatus including. in thisinstance; bulb 50 should exceed that of the trap system.

Furthermore, the gas detection system of Figure 2 includes four coils40c, 4011, 40c and 40f in place of the two coils 40a and 40b of Figurel. Coils c and 40d are disposed in a first bulb or com-- bustion chamber310 and coils 40c and 40, in a second and symmetrical bulb 31b. The twobulbs are connected by a conduit 52 surrounded by a metal tube or waterjacket 53, or other cooling element, so that the heat formed bycombustion on coils 40c and 40d does not affect coils 40c and 40 thepurpose of which is to balance out atmospheric effects, etc. Coils 40c,40d, 40c and 40) form the four arms or a Wheatstone bridge which can bebalanced by means of variable resistance 56. Indicating or recordingdevice 44, for instance a galvanometer or milliammeter protected byshunt resistance under the control of switch 58 is connected across onediagonal of the Wheatstone bridge while battery 42 under the control ofswitch 43 is connected across the other diagonal. Thus the reading ofindicating or recording device 44 reflects the temperature of cells 460and 40d and provides an indication of the amount of hydrocarbons,particularly ethanepentane hydrocarbons, present in the soil gassamples. The hydrocarbons passing through conduit 52 and cooled by tubeor jacket 53 impinge on coils 40c and 40} which are thus subjected tothe same conditions as coils 40c and 40d g lcgept for the removal ofcombustibles in bulb While I have described by invention in connectionwith certain preferred embodiments thereof, it is to be understood thatthese are by way of illustration and not by way of limitation and I donot mean to be restricted thereto but only to the scope of the appendedclaims in which I have endeavored to describe the novelty inherent in myinvention.

I claim:

1. A method of geochemical prospecting comprising forming an enlargedsoil gas collecting zone in the soil, withdrawing soil gas from saidsoil gas collecting zone through at least one low temperature trappingzone whereby soil hydrocarbons are condensed in said trapping zone,connecting a hydrocarbon measurement zone to said trapping zone, raisingthe temperature of said trapping zone to promote the vaporization ofsaid condensed soil gas hydrocarbons, passing an oxygen-containin gasthrough said trapping zone and into said hydrocarbon measurement zone tocarry said vaporized hydrocarbons into said hydrocarbon measurementzone, and measuring the comparative effects of a sample of said gascarrying said vaporized hydrocarbons and a sample of comparable gas freefrom said vaporized hydrocarbons on the resistance of hot wires havingsurfaces adapted to promote the surface oxidation of hydrocarbons,whereby the amount of said vaporized hydrocarbons is measured.

2. A method of geochemical prospecting comprising forming an enlargedsoil gas collecting zone in the soil, withdrawing soil gas from saidsoil gas collecting zone through at least one low temperature trappingzone whereby soil hydrocarbons are condensed in said trapping zone,discarding the remainder of said soil gases, connecting a sample storagezone to said trapping zone, raising the temperature of said trappingzone to promote the vaporization of said condensed soil gashydrocarbons, passing an oxygen-containing gas through said trappingzone and into said sample storage zone to carry said vaporizedhydroca'rbons into said sample storage zone, transferring gases fromsaid sample storage zone to a hydrocarbon measurement zone, andmeasuring the comparative effects of a sample of said gas carrying saidvaporized hydrocarbons and a sample of comparable gas free from saidvaporized hydrocarbons on the resistances of hot wires having surfacesadapted to promote the surface oxidation of hydrocarbons, whereby theamount of said vaporized hydrocarbons is measured.

A method of geochemical prospecting comprising removing soil gases fromsoil, trapping condensable hydrocarbons therefrom, conveying saidtrapped hydrocarbons to a hydrocarbon measurement zone in a sample ofoxygen-containing gas, and comparing the amounts of hydrocarbons in saidsample of gas and in a second sample of gas from the .same source butfree of said trapped hydrocarbons.

4. A method of geochemical prospecting comprising removing soil gasesfrom soil, trapping condensable hydrocarbons therefrom, vaporizing saidtrapped hydrocarbons, conveying said vaporized hydrocarbons to ahydrocarbon measurement zone in a sample of gas, collecting a secondsample of gas from the same source as said firstmentioned sample of gasin a second hydrocarbon measurement zone, and measuring in saidhydrocarbon measurement zones the difierential hydrocarbon contents ofthe gas samples contained therein.

5. A method of geochemical prospecting comprising removing soil gasesfrom soil, trapping condensable hydrocarbons therefrom, vaporizing saidtrapped hydrocarbons, conveying said vaporized hydrocarbons to ahydrocarbon measurement zone in a sample of oxygen-containing gas,conveying a second sample of gas from the same source as saidfirst-mentioned sample oi gas to a second hydrocarbon measurement zone,each of said hydrocarbon measurement zones containing a hot wire havinga surface containing at least one platinum group metal, and measuring insaid hydrocarbon measurement zones the differential efiects of thehydrocarbon contents of said two samples of gas on the resistances ofsaid hot wires.

6. A method of geochemical prospecting comprising forming anenlargedsoil gas collecting zone in the soil,' connecting at least onelow temperature trapping zone to said soil gas collecting zone,withdrawing soil gas from said soil gas collecting zone through said lowtemperature trapping zone whereby soil hydrocarbons are condensed insaid trapping zone, discarding the remainder of said soil gases,connecting a hydrocarbon measurement zone to said trapping zone, raisingthe temperature of said trapping zone to promote the vaporization ofsaid condensed soil gas hydrocarbons, passing gas from a source ofimiform oxygen-containing gas through said trapping zone and into saidhydrocarbon measurement zone, passing gas from said source into a secondhydrocarbon measurement zone, each of said hydrocarbon measurement zonescontaining a hot wire having a surface containing at least one platinumgroup metal, and measuring in said hydrocarbon measurement zones thedifierential efiects of the hydrocarbon contents of said two streams ofgas on the resistances of said hot wires.

7. A method of geochemical prospecting comprising forming an enlargedsoil gas sample collecting zone in the soil, sealing said samplecollecting zone from atmospheric air, Withdrawing soil gas from saidsample collecting zone through a trapping zone held at a low temperatureand thence through a second trapping zone maintained at a lowertemperature, said first-mentioned trapping zone being adapted to causethe precipitation of relatively high boiling impurities present in saidsoil gas and said second-mentioned trapping zone being adapted to causethe precipitation of light hydrocarbons heavier than methane, increasingthe temperature of said secand-mentioned trapping zone while maintainingsaid first-mentioned trapping zone at said low temperature, passingfurther soil gas from said soil gas collecting zone first through saidsecondmentioned trapping zone while at said increased temperature andthence through said first-mentoined trapping zone while at saidfirst-mentioned low temperature and thence to a hydrocarbon measurementzone, and measuring in said hydrocarbon measurement zone a function ofthe hydrocarbon content of the gases entering said hydrocarbonmeasurement zone.

8. A method of geochemical prospecting comprising forming an enlargedsoil gas sample collecting zone in the soil, sealing said collectingzone from atmospheric air, leaving said sample collecting zone in sealedcondition for a prolonged period of time sufhcient to achievesubstantial equilibrium between the gases present in said collectingzone and the gases present in the soi1 surrounding said collecting zone,attaching directly to said collecting zone a purification zone and atleast two trapping zones, one of said trapping zones being maintained ata low temperature in the vicinity of --80 C. and the second of saidtrapping zones being maintained at a still lower temperature in thevicinity of -196 0., withdrawing soil gas from said collecting zone andpassing said soil gas so withdrawn first through said purification zoneand thence in sequence through said first-mentioned trapping zone andsaid second trapping zone, said trapping zones being maintained at thetemperatures aforementioned, increasing the temperature of said secondtrapping zone to vaporize hydrocarbons condensed therein, passingfurther soil gas from said collecting zone in sequence through saidsecond trapping zone and said first-mentioned trapping zone while saidfirst-mentioned trapping zone is at said low temperature and said secondtrapping zone is at said increased temperature, passing saidlast-mentioned soil gas carrying hydrocarbons originally condensed insaid second trapping zone from said first-mentioned trapping zone to ahydrocarbon measurement zone, and measuring in said last-mentioned zonea function of the hydrocarbon content of the gases entering saidlast-mentioned zone.

9. In apparatus for soil gas analysis adapted foruse in the field, afirst low temperature trap, a second low temperature trap, a conduitconnecting said traps, a pair of conduits leading from a source of soilgas one to each of said traps, valve means disposed in each one of saidpair of conduits, valved vent means communicating with the conduitconnecting said second trap with said source of soil gas, meansconnected to the system for pumping soil gas through said traps andconduits, whereby soil gas may be passed in sequence through said firsttra then through said second trap and then vented, and wherebyadditional soil gas may be passed first through said second trap andthen through said first trap, and measuring means in valvedcommunication with said first trap for measuring the amount ofhydrocarbons contained in said last-mentioned gas.

10. In apparatus for soilgas analysis adapted for use in the field, afirst low temperature trap,

a second low temperature trap, a first conduit leading from a source oisoil gas to said first trap, a second conduit connecting said first andsecond traps, a third conduit leading from said second trap to thesource of soil gas, valves disposed in said first and third conduits,valved vent means in communication with said third conduit, meansconnected to the system for pumping soil gas through said traps andconduits, the operation oi said valves permitting soil gas to be passedin sequence through said first trap, then through said second trap, andthen vented and permitting additional soil gas to be passed through saidsecond trap and then through said first trap and measuring meanscommunicating with said first conduit for measuring the amount ofhydrocarbons contained in said last-mentioned gas.

11. Apparatus for geochemical prospecting by analyzing soil gaswithdrawn from a soil gas collecting chamber in the soil comprising atrapping system, a first conduit means leading from said collectingchamber to said trapping system, said trapping system including at leastone low temperature trap for condensing soil gas hydrocar= bons heavierthan methane, pumping means connected to said trapping system forwithdrawing soil gas from said collecting chamber and for passing saidsoil gas through said low temperature trap, a gas measurement chambercontaining a wire having an oxidative surface, a second conduit meansconnecting said gas measurement chamber with said first conduit means, asecond gas measurement chamber containing a second wire having anoxidative surface, a third conduit means connecting said second gasmeasurement chamber with said, second conduit means, twoway valvesdisposed at the junction of said first and second conduit means and atthe junction of said second and third conduit means, whereby thehydrocarbons heavier than methane may be passed in admixture with asample of soil gas from said low temperature trap to said first gasmeasurement chamber, and whereby a second sample 01 fresh soil gas maybe passed from said collecting chamber to said second gas measurementchamber, means for heating said wires and means for measuring therelative resistances of said wires.

12. Apparatus for geochemical prospecting comprising conduit meanscommunicating at one end with the soil for removing gas therefrom,extracting means communicating with the other end of said conduit meansfor extracting hydrocarbons from said soil gas, a first gas measurementchamber communicating with said extracting means and containing a wirehaving an oxidative surface whereby said extracted hydrocarbons inadmixture with a sample of soil gas may be passed from said extractingmeans to said first gas meas urement chamber, a second gas measurementchamber containing a second wire having an oxidative surface andcommunicating with said conduit means whereby a second sample of freshsoil gas may be introduced into said second gas measurement chamber,means for heating said wires and means for measuring the relativeresistances of said wires.

R. THOMAS SANDERSON.

